Many underbody parts and body structural members of automobiles have been produced by pressing steel sheets having predetermined strength. From the viewpoint of global environment conservation, weight lightening of automobile car bodies has recently been desired eagerly, and the effort to decrease the thickness of the steel sheet used by strengthening the steel sheet has been continued. However, the pressing workability decreases with strengthening of steel sheets, and thus it is often difficult to process steel sheets into desired member shapes.
Therefore, Patent Literature 1 proposes a processing technique referred to as “hot-pressing” which enables both easy working and strengthening by quenching and processing a heated steel sheet at the same time using a mold including a die and a punch. However, in the hot-pressing, the steel sheet is heated to a high temperature of about 950° C. before the hot-pressing, and thus scales (Fe oxides) are produced on a surface of the steel sheet and are separated during the hot-pressing, thereby causing the problem of damaging the mold or damaging a surface of a member after the hot-pressing. In addition, the scales remaining on a surface of the member causes a poor appearance, a decrease in coating adhesion, or a decrease in corrosion resistance after coating. Therefore, the scales on a surface of the member are generally removed by a treatment such as pickling or shot blasting, but this complicates the production process and decreases productivity.
From this viewpoint, there has been demand for a hot-pressing technique capable of suppressing the formation of scales during heating before hot-pressing and improving coating adhesion and corrosion resistance after coating of a member after the hot-pressing, and a steel sheet having a film such as a coating layer provided on a surface thereof, and a hot-pressing method using the steel sheet have been proposed.
For example, Patent Literature 2 discloses a coated steel sheet coated with Al or an Al alloy. It is described that by using the coated steel sheet, decarburization and oxidation are prevented during heating before hot-pressing, and a hot-pressed member having very high strength and excellent corrosion resistance can be produced.
In addition, Patent Literature 3 discloses a hot-pressing method in which when a steel sheet coated with Zn or a Zn-based alloy is hot-pressed, an alloy compound such as a Zn—Fe-based compound or Zn—Fe—Al-based compound, which prevents corrosion and decarburization and has a lubricating function, is produced on a surface of the steel sheet during heating before hot-pressing. It is also described that with a hot-pressed member produced by the method, particularly a hot-pressed member including a steel sheet coated with Zn-50 to 55 mass % Al, the excellent corrosion preventing effect can be achieved.
Further, Patent Literature 4 discloses a hot-pressing method including heating a steel sheet provided with a coating, which mainly contains Al or Zn, in an atmosphere having a hydrogen concentration of 6% by volume or less and a dew point of 10° C. or less at a heating temperature of an Ac3 transformation point or more and 1100° C. or less, and then hot-pressing the steel sheet, thereby achieving excellent hydrogen embrittlement resistance. In this hot-pressing method, the amounts of hydrogen and water vapor in the atmosphere during heating are decreased to decrease the amount of hydrogen entering the steel, thereby attempting to avoid hydrogen embrittlement associated with an increase in strength to over 1000 MPa.